Enhancements===========- Made dialogs appear centered on the main window.

- Added a window icon to all windows.

- Added a total price to the order list (request by Heiko Niemeier).

- Added a Preferences menu item, where it is possible to specify a conversion rate from US$ to ones local currency. This will then be used to display the total price on the order list in the local currency as well (request by Heiko Niemeier).